Presently, an optical fiber used for an optical fiber communication system spreading generally has a structure in which outer circumference of one core is surrounded by a clad and an optical signal is propagated through the core, so that information is transmitted. Recently, an amount of information transmitted increases drastically with the spread of the optical fiber communication system. According to the increase in the amount of information transmitted, in the optical fiber communication system, a large number of optical fibers such as dozens or hundreds of optical fibers are used and large-capacity long-distance optical communication is performed.
It is known that, in the optical fiber communication system, a multicore fiber in which outer circumferences of a plurality of cores are surrounded by one clad is used and a plurality of signals are transmitted using light propagated through the individual cores.
An example of the multicore fiber is described in Patent Literature 1 described below. In the multicore fiber, one core is disposed on a center of a clad and six cores are disposed around the core disposed on the center. Because such an arrangement is a structure in which the cores can be disposed in a closest packing state, a large number of cores can be disposed with respect to an outer diameter of a specific clad. In addition, in the multicore fiber described in Patent Literature 1, propagation constants of light propagated through the cores adjacent to each other are different from each other to suppress crosstalk of light propagated through the individual cores.
However, there is a request for suppressing the crosstalk more than the case in which effective refractive indexes of the cores adjacent to each other are changed like the multicore fiber described in Patent Literature 1. Therefore, a multicore fiber in which a low refractive index layer having a refractive index lower than a refractive index of the clad is disposed to surround an outer circumferential surface of each core and the crosstalk is further prevented is known. This multicore fiber is described in Patent Literature 2 described below. When the multicore fiber is viewed from a viewpoint of the refractive index, the low refractive index layer has a shape of a trench. For this reason, the multicore fiber is called a trench type and a configuration from the core to the low refractive index layer is called a core element. Even in the trench type multicore fiber, propagation constants of light propagated through the cores adjacent to each other are different from each other to suppress the crosstalk of light propagated through the individual cores.    [Patent Literature 1] JP-A-2011-170336    [Patent Literature 2] JP-A-2012-118495